예제 #1
0
/*
 * SetTransactionSnapshot
 *		Set the transaction's snapshot from an imported MVCC snapshot.
 *
 * Note that this is very closely tied to GetTransactionSnapshot --- it
 * must take care of all the same considerations as the first-snapshot case
 * in GetTransactionSnapshot.
 */
static void
SetTransactionSnapshot(Snapshot sourcesnap, TransactionId sourcexid)
{
	/* Caller should have checked this already */
	Assert(!FirstSnapshotSet);

	Assert(RegisteredSnapshots == 0);
	Assert(FirstXactSnapshot == NULL);

	/*
	 * Even though we are not going to use the snapshot it computes, we must
	 * call GetSnapshotData, for two reasons: (1) to be sure that
	 * CurrentSnapshotData's XID arrays have been allocated, and (2) to update
	 * RecentXmin and RecentGlobalXmin.  (We could alternatively include those
	 * two variables in exported snapshot files, but it seems better to have
	 * snapshot importers compute reasonably up-to-date values for them.)
	 */
	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);

	/*
	 * Now copy appropriate fields from the source snapshot.
	 */
	CurrentSnapshot->xmin = sourcesnap->xmin;
	CurrentSnapshot->xmax = sourcesnap->xmax;
	CurrentSnapshot->xcnt = sourcesnap->xcnt;
	Assert(sourcesnap->xcnt <= GetMaxSnapshotXidCount());
	memcpy(CurrentSnapshot->xip, sourcesnap->xip,
		   sourcesnap->xcnt * sizeof(TransactionId));
	CurrentSnapshot->subxcnt = sourcesnap->subxcnt;
	Assert(sourcesnap->subxcnt <= GetMaxSnapshotSubxidCount());
	memcpy(CurrentSnapshot->subxip, sourcesnap->subxip,
		   sourcesnap->subxcnt * sizeof(TransactionId));
	CurrentSnapshot->suboverflowed = sourcesnap->suboverflowed;
	CurrentSnapshot->takenDuringRecovery = sourcesnap->takenDuringRecovery;
	/* NB: curcid should NOT be copied, it's a local matter */

	/*
	 * Now we have to fix what GetSnapshotData did with MyProc->xmin and
	 * TransactionXmin.  There is a race condition: to make sure we are not
	 * causing the global xmin to go backwards, we have to test that the
	 * source transaction is still running, and that has to be done atomically.
	 * So let procarray.c do it.
	 *
	 * Note: in serializable mode, predicate.c will do this a second time.
	 * It doesn't seem worth contorting the logic here to avoid two calls,
	 * especially since it's not clear that predicate.c *must* do this.
	 */
	if (!ProcArrayInstallImportedXmin(CurrentSnapshot->xmin, sourcexid))
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("could not import the requested snapshot"),
				 errdetail("The source transaction %u is not running anymore.",
						   sourcexid)));

	/*
	 * In transaction-snapshot mode, the first snapshot must live until end of
	 * xact, so we must make a copy of it.  Furthermore, if we're running in
	 * serializable mode, predicate.c needs to do its own processing.
	 */
	if (IsolationUsesXactSnapshot())
	{
		if (IsolationIsSerializable())
			SetSerializableTransactionSnapshot(CurrentSnapshot, sourcexid);
		/* Make a saved copy */
		CurrentSnapshot = CopySnapshot(CurrentSnapshot);
		FirstXactSnapshot = CurrentSnapshot;
		/* Mark it as "registered" in FirstXactSnapshot */
		FirstXactSnapshot->regd_count++;
		RegisteredSnapshots++;
	}

	FirstSnapshotSet = true;
}
예제 #2
0
/*
 * ImportSnapshot
 *      Import a previously exported snapshot.  The argument should be a
 *      filename in SNAPSHOT_EXPORT_DIR.  Load the snapshot from that file.
 *      This is called by "SET TRANSACTION SNAPSHOT 'foo'".
 */
void
ImportSnapshot(const char *idstr)
{
	char		path[MAXPGPATH];
	FILE	   *f;
	struct stat	stat_buf;
	char	   *filebuf;
	int			xcnt;
	int			i;
	TransactionId src_xid;
	Oid			src_dbid;
	int			src_isolevel;
	bool		src_readonly;
	SnapshotData snapshot;

	/*
	 * Must be at top level of a fresh transaction.  Note in particular that
	 * we check we haven't acquired an XID --- if we have, it's conceivable
	 * that the snapshot would show it as not running, making for very
	 * screwy behavior.
	 */
	if (FirstSnapshotSet ||
		GetTopTransactionIdIfAny() != InvalidTransactionId ||
		IsSubTransaction())
		ereport(ERROR,
				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
				 errmsg("SET TRANSACTION SNAPSHOT must be called before any query")));

	/*
	 * If we are in read committed mode then the next query would execute
	 * with a new snapshot thus making this function call quite useless.
	 */
	if (!IsolationUsesXactSnapshot())
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ")));

	/*
	 * Verify the identifier: only 0-9, A-F and hyphens are allowed.  We do
	 * this mainly to prevent reading arbitrary files.
	 */
	if (strspn(idstr, "0123456789ABCDEF-") != strlen(idstr))
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
				 errmsg("invalid snapshot identifier \"%s\"", idstr)));

	/* OK, read the file */
	snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", idstr);

	f = AllocateFile(path, PG_BINARY_R);
	if (!f)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
				 errmsg("invalid snapshot identifier \"%s\"", idstr)));

	/* get the size of the file so that we know how much memory we need */
	if (fstat(fileno(f), &stat_buf))
		elog(ERROR, "could not stat file \"%s\": %m", path);

	/* and read the file into a palloc'd string */
	filebuf = (char *) palloc(stat_buf.st_size + 1);
	if (fread(filebuf, stat_buf.st_size, 1, f) != 1)
		elog(ERROR, "could not read file \"%s\": %m", path);

	filebuf[stat_buf.st_size] = '\0';

	FreeFile(f);

	/*
	 * Construct a snapshot struct by parsing the file content.
	 */
	memset(&snapshot, 0, sizeof(snapshot));

	src_xid = parseXidFromText("xid:", &filebuf, path);
	/* we abuse parseXidFromText a bit here ... */
	src_dbid = parseXidFromText("dbid:", &filebuf, path);
	src_isolevel = parseIntFromText("iso:", &filebuf, path);
	src_readonly = parseIntFromText("ro:", &filebuf, path);

	snapshot.xmin = parseXidFromText("xmin:", &filebuf, path);
	snapshot.xmax = parseXidFromText("xmax:", &filebuf, path);

	snapshot.xcnt = xcnt = parseIntFromText("xcnt:", &filebuf, path);

	/* sanity-check the xid count before palloc */
	if (xcnt < 0 || xcnt > GetMaxSnapshotXidCount())
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
				 errmsg("invalid snapshot data in file \"%s\"", path)));

	snapshot.xip = (TransactionId *) palloc(xcnt * sizeof(TransactionId));
	for (i = 0; i < xcnt; i++)
		snapshot.xip[i] = parseXidFromText("xip:", &filebuf, path);

	snapshot.suboverflowed = parseIntFromText("sof:", &filebuf, path);

	if (!snapshot.suboverflowed)
	{
		snapshot.subxcnt = xcnt = parseIntFromText("sxcnt:", &filebuf, path);

		/* sanity-check the xid count before palloc */
		if (xcnt < 0 || xcnt > GetMaxSnapshotSubxidCount())
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
					 errmsg("invalid snapshot data in file \"%s\"", path)));

		snapshot.subxip = (TransactionId *) palloc(xcnt * sizeof(TransactionId));
		for (i = 0; i < xcnt; i++)
			snapshot.subxip[i] = parseXidFromText("sxp:", &filebuf, path);
	}
	else
	{
		snapshot.subxcnt = 0;
		snapshot.subxip = NULL;
	}

	snapshot.takenDuringRecovery = parseIntFromText("rec:", &filebuf, path);

	/*
	 * Do some additional sanity checking, just to protect ourselves.  We
	 * don't trouble to check the array elements, just the most critical
	 * fields.
	 */
	if (!TransactionIdIsNormal(src_xid) ||
		!OidIsValid(src_dbid) ||
		!TransactionIdIsNormal(snapshot.xmin) ||
		!TransactionIdIsNormal(snapshot.xmax))
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
				 errmsg("invalid snapshot data in file \"%s\"", path)));

	/*
	 * If we're serializable, the source transaction must be too, otherwise
	 * predicate.c has problems (SxactGlobalXmin could go backwards).  Also,
	 * a non-read-only transaction can't adopt a snapshot from a read-only
	 * transaction, as predicate.c handles the cases very differently.
	 */
	if (IsolationIsSerializable())
	{
		if (src_isolevel != XACT_SERIALIZABLE)
			ereport(ERROR,
					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
					 errmsg("a serializable transaction cannot import a snapshot from a non-serializable transaction")));
		if (src_readonly && !XactReadOnly)
			ereport(ERROR,
					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
					 errmsg("a non-read-only serializable transaction cannot import a snapshot from a read-only transaction")));
	}

	/*
	 * We cannot import a snapshot that was taken in a different database,
	 * because vacuum calculates OldestXmin on a per-database basis; so the
	 * source transaction's xmin doesn't protect us from data loss.  This
	 * restriction could be removed if the source transaction were to mark
	 * its xmin as being globally applicable.  But that would require some
	 * additional syntax, since that has to be known when the snapshot is
	 * initially taken.  (See pgsql-hackers discussion of 2011-10-21.)
	 */
	if (src_dbid != MyDatabaseId)
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("cannot import a snapshot from a different database")));

	/* OK, install the snapshot */
	SetTransactionSnapshot(&snapshot, src_xid);
}
예제 #3
0
/* Merge local and global snapshots.
 * Produce most restricted (conservative) snapshot which treate transaction as in-progress if is is marked as in-progress
 * either in local, either in global snapshots
 */
static void DtmMergeWithGlobalSnapshot(Snapshot dst)
{
	int i, j, n;
	TransactionId xid;
	Snapshot src = &DtmSnapshot;

	if (!(TransactionIdIsValid(src->xmin) && TransactionIdIsValid(src->xmax))) { 
        PgGetSnapshotData(dst);
        return;
    }

GetLocalSnapshot:
	/*
	 * Check that global and local snapshots are consistent: transactions marked as completed in global snapshot
	 * should be completed locally
	 */
	dst = PgGetSnapshotData(dst);
	for (i = 0; i < dst->xcnt; i++)
		if (TransactionIdIsInDoubt(dst->xip[i]))
			goto GetLocalSnapshot;
	for (xid = dst->xmax; xid < src->xmax; xid++)
		if (TransactionIdIsInDoubt(xid))
			goto GetLocalSnapshot;

    GetCurrentTransactionId();
    DumpSnapshot(dst, "local");
	DumpSnapshot(src, "DTM");

	if (src->xmax < dst->xmax) dst->xmax = src->xmax;
	if (src->xmin < dst->xmin) dst->xmin = src->xmin;
	Assert(src->subxcnt == 0);

	if (src->xcnt + dst->subxcnt + dst->xcnt <= GetMaxSnapshotXidCount())
	{
		Assert(dst->subxcnt == 0);
		memcpy(dst->xip + dst->xcnt, src->xip, src->xcnt*sizeof(TransactionId));
		n = dst->xcnt + src->xcnt;

		qsort(dst->xip, n, sizeof(TransactionId), xidComparator);
		xid = InvalidTransactionId;

		for (i = 0, j = 0; i < n && dst->xip[i] < dst->xmax; i++)
			if (dst->xip[i] != xid)
				dst->xip[j++] = xid = dst->xip[i];
		dst->xcnt = j;
	}
	else
	{
		Assert(src->xcnt + dst->subxcnt + dst->xcnt <= GetMaxSnapshotSubxidCount());
		memcpy(dst->subxip + dst->subxcnt, dst->xip, dst->xcnt*sizeof(TransactionId));
		memcpy(dst->subxip + dst->subxcnt + dst->xcnt, src->xip, src->xcnt*sizeof(TransactionId));
		n = dst->xcnt + dst->subxcnt + src->xcnt;

		qsort(dst->subxip, n, sizeof(TransactionId), xidComparator);
		xid = InvalidTransactionId;

		for (i = 0, j = 0; i < n && dst->subxip[i] < dst->xmax; i++)
			if (dst->subxip[i] != xid)
				dst->subxip[j++] = xid = dst->subxip[i];
		dst->subxcnt = j;
		dst->xcnt = 0;
	}
	DumpSnapshot(dst, "merged");
}